In the case of internal combustion engines, catalytic converters are used in the exhaust gas manifold for cleaning the exhaust gases in order to comply with the emission specifications. Said motor control system has a temperature model for monitoring the temperature of the catalytic converter, which calculates the exhaust gas temperature and/or the temperature of the catalytic converter. Depending on the calculated temperature, cooling measures are initiated, in the event of the temperature being too high, to protect the catalytic converter. Such cooling measures consist of a change in the air-to-fuel ratio towards a fuel excess, namely a so-called process of making the mixture slightly richer. A regulator takes charge of said process of making the mixture slightly richer at the inlet of which the difference between the simulated and the maximum permissible temperature of the catalytic converter is provided. In the case of this basic approach, the problem is that the actual temperature of the catalytic converter only reacts to the slightly richer mixture after a considerable delay. Therefore, the regulator has a long control path. The temperature model in the motor control system takes account of this long control path and copies the delayed behavior of the catalytic converter. Depending on the selected control parameters, this brings about either an oscillation condition of the regulator or an overswing of the temperature of the catalytic converter when the regulator is activated for the first time.
WO 03029634 discloses a method in which a lambda value, depending on a modulated or a measured temperature, is adjusted at least at one critical point of the exhaust gas system, deviating from the normal operation, to a temperature-dependent lambda value, in such a way that an exhaust gas temperature is decreased if the temperature determined exceeds a predetermined temperature value.
A method for controlling a component protection function of a catalytic converter is known from DE 102 01 465 B4. For this purpose, the modulated exhaust gas temperature is taken as a function of the lambda value and of further variables. By using the inverse function it is then possible to calculate, for a maximum temperature value, a lambda desired value for the purpose of component protection. Should it be the case that the component protection function is activated during the operation of an internal combustion engine, the lambda value will then be set at the lambda desired value calculated in this way. The problem with this method is the fact that the exhaust gas temperature model can only be inverted under certain assumptions. If the temperature change in the catalytic converter is also taken into account in the temperature model because of the exothermal chemical reactions, then there is no simple relationship which allows the inverting of the function. At most there is then a bijective relationship between the temperature and the lambda value.